Establishment of delayed hypersensitivity to DNCB results in enhanced growth of subsequent tumor. Preliminary results with nude mice and a non-immunogenic congener of DNCB suggest that T cells mediate this enhancement. When tumor inoculation precedes DNCB sensitization, mice are anergic to DNCB challenge 10 but not 7 days after sensitization. We also found that spleen cells of anergic tumorous mice contained DNCB reactive lymphocytes when tested by transfer to normal mice. Tumor-free DNCB sensitized mice did not develop such cells until much later.